Field of the Invention
The present invention relates to an image forming apparatus.
Description of the Related Art
There has been an image forming apparatus such as a copying machine and a printer in which a toner image (an image) formed by an image forming unit is fixed on a sheet and then the sheet is discharged by a discharge roller pair formed of two discharge rollers. There has been another image forming apparatus in which a sheet, on which a toner image is fixed, is conveyed again to an image forming unit and an image is formed on the other side of the sheet.
In such an image forming apparatus, when an image is formed on the sheet, an electrostatic latent image is first formed on the surface of a photosensitive drum provided on the image forming unit and developed by a toner to form a toner image on the surface of the photosensitive drum. Then, the toner image is transferred onto the sheet and then the sheet is heated and pressed in a fixing unit to fix the toner image onto the sheet. A discharge roller pair is rotated forward to discharge the sheet, on which the toner image is fixed, to a sheet stacking unit. When images are formed on both sides of a sheet, the discharge roller pair is reversely rotated to convey the sheet to a re-conveyance path and again to the image forming apparatus.
In recent years, a processing speed of an image forming apparatus has been increased and the size thereof has been reduced. For this reason, a distance between a fixing unit and a discharge roller pair is decreased and a sheet which is heated and pressed by the fixing unit to be extremely high in temperature is nipped by the discharge roller pair with the sheet remained at high temperature. This may cause the toner high in temperature to stick to the surface of the discharge roller. The toner sticking thereto further sticks to the sheet to be discharged next as a roller trace, which may degrade the image quality of the next sheet.
Conventionally, a material such as fluororesin high in resistance to toner fusion has been employed as a material of a discharge roller which is brought into pressure contact with the surface of the toner image to prevent a roller trace from occurring. If the processing speed of the image forming apparatus is further increased and the size thereof is further reduced, the roller trace cannot be prevented from occurring only by increasing resistance to toner fusion on the surface of the discharge roller. There is a method for preventing the roller trace from occurring in which a plurality of discharge rollers brought into pressure contact with a discharge roller is symmetrically tilted with respect to the direction in which the sheet is conveyed with the center of the sheet in the width direction thereof orthogonal to the sheet conveyance direction as a center. The discharge rollers are thus tilted to enable the rear end portion of the sheet to shave off the toner sticking to the surface of the discharge roller even if the toner at the leading edge of the sheet sticks to the surface of the discharge roller.
However, in a case where the discharge rollers are thus tilted, a problem is caused in which the end portion of the discharge roller is brought into close contact with a sheet to produce a linear trace on the sheet. The problem of the linear trace can be improved by decreasing the nip pressure of the discharge roller pair. However, it is difficult to decrease the nip pressure of the discharge roller pair because the re-conveyance path and a shutter mechanism for correcting the registration of the sheet are subjected to resistance when images are formed on both sides of a sheet.
Japanese Patent Application Laid-Open No. 2001-142328 discusses a technique in which the nip of a sub-conveyance member at the rear of the fixing unit is made attachable and detachable using an actuator such as a solenoid to solve both problems of the linear trace and required conveyance force to the re-conveyance path. However, with the structure discussed in Japanese Patent Application Laid-Open No. 2001-142328, the actuator is required, which remains a problem that the image forming apparatus is increased in size and the cost is increased because the number of components is increased.